1. Field of the Invention
This invention relates generally to telecommunication systems and assemblies, and more particularly to an AOTF and a system that uses at least one AOTF.
2. Description of Related Art
In modem telecommunication systems, many operations with digital signals are performed on an optical layer. For example, digital signals are optically amplified, multiplexed and demultiplexed. In long fiber transmission lines, the amplification function is performed by Erbium Doped Fiber Amplifiers (EDFA""s). The amplifier is able to compensate for power loss related to signal absorption, but it is unable to correct the signal distortion caused by linear dispersion, 4-wave mixing, polarization distortion and other propagation effects, and to get rid of noise accumulation along the transmission line. For these reasons, after the cascade of multiple amplifiers the optical signal has to be regenerated every few hundred kilometers. In practice, the regeneration is performed with electronic repeaters using optical-to-electronic conversion. However to decrease system cost and improve its reliability it is desirable to develop a system and a method of regeneration, or signal refreshing, without optical to electronic conversion. An optical repeater that amplifies and reshapes an input pulse without converting the pulse into the electrical domain is disclosed, for example, in the U.S. Pat. No. 4,971,417, xe2x80x9cRadiation-Hardened Optical Repeaterxe2x80x9d. The repeater comprises an optical gain device and an optical thresholding material producing the output signal when the intensity of the signal exceeds a threshold. The optical thresholding material such as polydiacetylene thereby performs a pulse shaping function. The nonlinear parameters of polydiacetylene are still under investigation, and its ability to function in an optically thresholding device has to be confirmed.
Another function vital to the telecommunication systems currently performed electronically is signal switching. The switching function is next to be performed on the optical level, especially in the Wavelength Division Multiplexing (WDM) systems. There are two types of optical switches currently under consideration. First, there are wavelength insensitive fiber-to-fiber switches. These switches (mechanical, thermo and electro-optical etc.) are dedicated to redirect the traffic from one optical fiber to another, and will be primarily used for network restoration and reconfiguration. For these purposes, the switching time of about 1 msec (typical for most of these switches) is adequate; however the existing switches do not satisfy the requirements for low cost, reliability and low insertion loss. Second, there are wavelength sensitive switches for WDM systems. In dense WDM systems having a small channel separation, the optical switching is seen as a wavelength sensitive procedure. A small fraction of the traffic carried by specific wavelength should be dropped and added at the intermediate communication node, with the rest of the traffic redirected to different fibers without optical to electronic conversion. This functionality promises significant cost saving in the future networks. Existing wavelength sensitive optical switches are usually bulky, power-consuming and introduce significant loss related to fiber-to-chip mode conversion. Mechanical switches interrupt the traffic stream during the switching time. Acousto-optic tunable filters, made in bulk optic or integrated optic forms, (AOTFs) where the WDM channels are split off by coherent interaction of the acoustic and optical fields though fast, less than about 1microsecond, are polarization and temperature dependent. Furthermore, the best AOTF consumes several watts of RF power, has spectral resolution about 3nm between the adjacent channels (which is not adequate for current WDM requirements), and introduces over 5 dB loss because of fiber-to-chip mode conversions.
Another wavelength-sensitive optical switch may be implemented with a tunable Fabry Perot filter (TFPF). When the filter is aligned to a specific wavelength, it is transparent to the incoming optical power. Though the filter mirrors are almost 100% reflective no power is reflected back from the filter. With the wavelength changed or the filter detuned (for example, by tilting the back mirror), the filter becomes almost totally reflective. With the optical circulator in front of the filter, the reflected power may be redirected from the incident port. The most advanced TFPF with mirrors built into the fiber and PZT alignment actuators have only 0.8 dB loss. The disadvantage of these filters is a need for active feedback and a reference element for frequency stability.
There is a need for an improved acousto-optic filter as well as communication systems with improved acousto-optic filters.
Accordingly, an object of the present invention is to provide an improved acousto-optic filter.
Another object of the present invention is to provide a tunable acousto-optic filter.
A further object of the present invention is to provide a tunable acousto-optic filter that includes a single mode optical fiber.
Still another object of the present invention is to provide an acoustic optic filter with multiple acoustic signals that have individual controllable strengths and frequencies.
Yet another object of the present invention is to provide an acoustic optic filter with multiple acoustic signals that have individual controllable strengths and frequencies, and each of the acoustic signals provides a coupling between the core mode and a different cladding mode.
Another object of the present invention is to provide an acousto-optic filter that includes an acoustic wave generator and an optical fiber with a core and a cladding, and a wavelength of an optical signal coupled to the cladding from the core is changed by varying the frequency of a signal applied to the acoustic wave generator.
Yet a further object of the present invention is to provide an acousto-optic filter that includes an optical fiber with a core and a cladding where a strength or magnitude of an optical signal coupled from the core to the cladding is changed by varying the amplitude of a signal applied to an acousto-optical wave.
These and other objects of the invention are achieved in an acousto-optic filter that includes a non-birefringent single mode optical fiber with a longitudinal axis, a core and a cladding in a surrounding relationship to the core. The optical fiber has multiple cladding modes and a single core mode that is guided along the core. An acoustic wave propagation member has a proximal end and a distal end coupled to the optical fiber. The acoustic wave propagation member propagates an acoustic wave from the proximal to the distal end and launches a flexural wave in the optical fiber. At least one acoustic wave generator is coupled to the proximal end of the acoustic wave propagation member.
In another embodiment of the present invention, a heatsink is coupled to the acoustic wave generator. The acoustic wave generator is positioned between the acoustic wave propagation member and the heatsink.
In yet another embodiment of the present invention, an acoustic damper is positioned at a distal portion of the optical fiber. An interactive region extends from the distal end of the acoustic wave propagation member to a proximal portion of the acoustic damper.
In another embodiment of the present invention, an optical communication system is provided with a transmitter. The transmitter includes an acoustic-optic filter that has a single mode optical fiber with a longitudinal axis, a core and a cladding in a surrounding relationship to the core. The optical fiber has multiple cladding modes and a single core mode guided along the core. An acoustic wave propagation member is included with a proximal end and a distal end. The distal end of the acoustic wave propagation member is coupled to the optical fiber. The acoustic wave propagation member propagates an acoustic wave from the proximal to the distal end and launches a flexural wave in the optical fiber. At least one acoustic wave generator is coupled to the proximal end of the acoustic wave propagation member. A receiver is coupled to the transmitter.
In another embodiment of the present invention, an optical communication system is provided with a transmitter and a receiver. The receiver includes an acoustic-optic filter that has a single mode optical fiber with a longitudinal axis, a core and a cladding in a surrounding relationship to the core. The optical fiber has multiple cladding modes and a single core mode guided along the core. An acoustic wave propagation member is included with a proximal end and a distal end. The distal end of the acoustic wave propagation member is coupled to the optical fiber. The acoustic wave propagation member propagates an acoustic wave from the proximal to the distal end and launches a flexural wave in the optical fiber. At least one acoustic wave generator is coupled to the proximal end of the acoustic wave propagation member.
In another embodiment of the present invention, an optical communication system is provided with a transmitter coupled to a receiver. The receiver includes an acoustic-optic filter that has a single mode optical fiber with a longitudinal axis, a core and a cladding in a surrounding relationship to the core. The optical fiber has multiple cladding modes and a single core mode guided along the core. An acoustic wave propagation member is included with a proximal end and a distal end. The distal end of the acoustic wave propagation member is coupled to the optical fiber. The acoustic wave propagation member propagates an acoustic wave from the proximal to the distal end and launches a flexural wave in the optical fiber. At least one acoustic wave generator is coupled to the proximal end of the acoustic wave propagation member.